Interpretive Summary: The standard method used to determine fecal contamination of recreational water supplies is the membrane filter (M-FC)technique. This method provides an estimate of fecal coliforms, of which a certain number of positive colonies must be further examined for identification. The public sometimes assumes that positive fecal coliform samples equate to E. coli contamination, which is not always the case. This study examined the various methods used to identify the genus of positive M-FC colonies to demonstrate the importance of choosing methods that provide color changes, rather than identifying a specific color for positive results.

Technical Abstract:
The fecal coliform, Escherichia coli, is a historical organism for the detection of fecal pollution in water supplies. The presence of E. coli indicates a potential contamination of the water supply by other more hazardous human pathogens. In order to accurately determine the presence and degree of fecal contamination, it is important that standard methods approved by the US Environmental Protection Agency (US EPA) are designed to determine the presence of E. coli in a water supply, and distinguish E. coli from other coliform bacteria (e.g. Citrobacter, Klebsiella and Enterobacter). These genera of bacteria are present not only in fecal matter, but also in soil and runoff water and are not good indicators of fecal contamination. There is also ambiguity in determining a positive result for fecal coliforms on MF-C filters by a blue colony. When all variations of blue, including light blue or glossy blue, were examined, confirmation methods agreed with the positive M-FC result less often than when colonies that the technician would merely call “blue”, with no descriptors, were examined. Approximately 48% of MF-C positive colonies were found to be E. coli with 4 methylumbelliferyl-ß-D-glucuronide (MUG), and only 23% of samples producing a positive result on M-FC media were found to be E. coli using API-20E test strips and current API-20E profiles. The majority of other M-FC blue colonies were found to be Klebsiella or unidentifiable with current API-20E profiles. Two positive M-FC colonies were found to be Kluyvera with API-20E, both of which cleaved MUG producing fluorescence under UV light, a characteristic used to differentiate E. coli from other fecal coliforms.